While a pure electric vehicle uses a battery as a power source, a hybrid-engine electric vehicle and a fuel-cell hybrid electric vehicle use a battery as a energy buffer. In these electric vehicles, the battery is an important device that directly relates to the efficiency of the vehicle.
A battery management system (BMS) manages overall states of the battery, and controls the battery so that it operates in an optimal condition to maximize its lifespan. The battery management system supports charge generation control and driving control by providing state of charge (SOC) information to a vehicle controller that maintains overall control of the electric vehicle. Generally, the state of charge of a battery is defined to be the ratio of the remaining capacity of the battery to the fully-charged capacity of the battery. If the state of charge information is inaccurate, i.e., if it contains errors, the ability of the battery management system to effectively manage and optimize the battery operation is reduced.
Examples of functions of the battery management system include: (1) estimating a state of charge of the battery; (2) detecting a full charge; (3) maintaining an equilibrium of voltages between cell modules of the battery; (4) controlling maximum charge and discharge voltages in accordance with the state of charge (SOC) of the battery; and (5) performing battery safety and cooling control.
To calculate a current state of charge of the battery, a battery management system generally measures the amount of charge current and discharge current. To make these calculations, an analog signal, which is detected by a current sensor, must be converted to a digital signal. During the conversion, an error corresponding to the amount of current measured, and hence the state of charge, typically accumulates. This error can lead to inaccurate management and control by the battery management system. To reduce the cumulative error, better A/D converters and increased-accuracy current sensors must be used, increasing the cost of the system.
Other sources of error for the state of charge measurement include self-discharge of battery voltage through internal battery resistance, and temperature inefficiencies of the battery.
In a pure electric vehicle, which applies a standardized charging method and uses the battery as an energy source, the SOC is reset to 100% after the battery is fully charged, thus reducing the effect of the cumulative error even though the error remains after the full charge.
However, in a hybrid-engine electric vehicle and a fuel-cell hybrid electric vehicle, which use the battery as an energy buffer, the battery is not fully charged at certain periods, and must continuously operate within a specific range of SOC. Yet, in these vehicles, the SOC of the battery is determined by various parameters that are dynamically changing according to various conditions. Consequently, it is difficult to estimate the SOC of the battery, and the cumulative error increases as the operating time of the battery increases. If the cumulative error is not compensated for, the battery may operate beyond an operating range of the SOC, potentially reducing the lifespan of the battery and decreasing the energy efficiency of the battery. Also, the voltage of the battery may become larger than a maximum-allowed system operating voltage.
The Korean Utility Model application no. 10-2000-82936 teaches a method to estimate a state of charge through battery charge/discharge terminal voltages, based on an inherent internal resistance of the battery according to the state of charge. However, because hybrid electrical systems have a dynamic charge/discharge cycle, this method is not well-suited to hybrid electric vehicles.
The information disclosed in this Background section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or suggestion that this information forms the prior art that is already known to a person skilled in the art.